1. Field of the Invention
The present invention relates to a misfire detecting device for an internal combustion engine equipped with a double-ended DLI (distributorless ignition system).
2. Description of the Prior Art
An ignition system of the type having a distributor is shown in FIG. 21. The distributor type ignition system includes an ignition coil 901, a battery 903 and a power transistor 904 both connected to a primary winding 902 of the ignition coil 901, and an engine control unit (ECU) 905 for supplying an ignition signal to the power transistor 904, a distributor 907 for distribution of a high voltage induced in a secondary winding 906, and spark plugs 908.about.911 connected to side electrodes of the distributor 907.
As shown in FIG. 21, a misfire detecting device consisting of a voltage dividing circuit 912 for dividing a plug voltage between a center electrode and an outer electrode and a misfire detecting circuit 913 for detecting a misfire of a spark plug at each engine cylinder on the basis of an attenuation characteristic of the plug voltage is incorporated in the above described prior art ignition system. In the meantime, since a positive potential at the electrode of the spark plug makes it possible to attain a smaller electrical resistance between the center electrode and the outer electrode at normal combustion, i.e., at normal firing and therefore makes it possible to attain judgment of the attenuation characteristic of the plug voltage with ease, than the negative potential does, the connection of the ignition coil 910 is reversed to the usual.
On the other hand, a double-ended distributorless ignition system has recently been used in a great number. The ignition system, as shown in FIG. 22, consists of ignition coils 920 and 921 for simultaneous ignition or spark, power transistors 924 and 925 for intermittently supplying battery current to primary windings 922 and 923 of the ignition coils 920 and 921, an electronic control unit (ECU) 926 for delivering an ignition signal to the power transistors 924 and 925, and spark plugs 927.about.930. In the meantime, the distributorless ignition system does not utilize a distributor and thus can reduce the radio noise and the cost.
It was revealed that when the double-ended distributorless ignition system was provided with a similar misfire detecting device consisting of a voltage dividing circuit 912, diodes 931, and a misfire detecting circuit 913, it was encountered by a following disadvantage.
The distributorless ignition system shown in FIG. 22 is so structured as to apply a high negative voltage to the center electrodes of the spark plugs 928 and 930. The spark plug whose center electrode is at negative potential, maintains a high electrical resistance between the center electrode and the outer electrode or ground electrode even after normal combustion or firing, so a remarkable or prominent attenuation of the plug voltage does not occur and therefore there occurs such a case in which the attenuation characteristic of the plug voltage in case of normal firing does not differ so largely from that in case of occurrence of a misfire.
Due to this, there is caused a difficulty in detecting a misfire at an engine cylinder having installed therein a spark plug the center electrode of which is at a negative potential.
In order to solve this problem, a following technique has been proposed as described in Japanese Patent Provisional Publication No. 4-179864.
As shown in FIG. 23, a secondary winding 941 is connected at opposite ends thereof by spark plugs 942 and 943, a positive polarity bias 947 of about 300 volts is always applied by way of a resistor 945 and a diode 946 to the positive polarity terminal 944 of the secondary winding 941, and a voltage at the output terminal 948 is detected for thereby determining or knowing combustion within a cylinder, i.e., occurrence of a misfire at a cylinder.
However, the technique described in the above patent publication is encountered by a following disadvantage.
Since the voltage of the positive polarity bias 947 is low because it is about 300 volts, there may occur such a case in which if there is a contact defect in a distribution line such as a high tension cable, a plug cap, etc., the bias voltage cannot go over the defective place though the high voltage for causing spark can go over it, thus making it difficult to ascertain the combustion within each cylinder, i.e., occurrence of a misfire of a spark plug at each cylinder.
Since the structure is such that the positive polarity bias 947 is always applied, it is required, when to detect the combustion, i.e., occurrence of a misfire at a particular period of a combustion cycle, to carry out a waveform treatment (i.e., integration or masking over a certain interval) of the voltage at the output terminal 948.